Buoy based marine seismic survey system and method

ABSTRACT

A seismic survey system for recording seismic data underwater in the presence of underwater currents. The system includes first plural buoys configured to descend in water at a predetermined depth (H 1 ) and each having a seismic receiver for recording the seismic data; a first vessel configured to launch the first plural buoys along a first line; and a second vessel configured to recover the first plural buoys at a second line, wherein there is a predetermined distance between the first and second lines. The first plural buoys are configured to travel underwater, at substantially the first predetermined depth (H 1 ), from the first line to the second line, due exclusively to the underwater currents.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is related to and claims the benefit of priorityof U.S. Provisional Application Ser. No. 61/588,741, filed on Jan. 20,2012, having the title “Method and System For Marine Seismic Survey,”and U.S. Provisional Application Ser. No. 61/619,013, filed on Apr. 2,2012, having the title “Marine Acquisition Using Subaquatic SeismicNodes,” the entire contents of which are incorporated herein byreference.

BACKGROUND

1. Technical Field

Embodiments of the subject matter disclosed herein generally relate tomethods and systems and, more particularly, to mechanisms and techniquesfor performing a marine seismic survey using buoys that carryappropriate seismic sensors.

2. Discussion of the Background

Marine seismic data acquisition and processing generate a profile(image) of geophysical structures under the seafloor. While this profiledoes not provide an accurate location of oil and gas reservoirs, itsuggests, to those trained in the field, the presence or absence ofthese reservoirs. Thus, providing a high-resolution image of thegeophysical structures under the seafloor is an ongoing process.

Reflection seismology is a method of geophysical exploration fordetermining properties of earth's subsurface, which is especiallyhelpful in the oil and gas industry. Marine reflection seismology isbased on using a controlled source of energy that sends the energy intothe earth. By measuring the time it takes for the reflections to comeback to plural receivers, it is possible to evaluate the depth offeatures causing such reflections. These features may be associated withsubterranean hydrocarbon deposits.

A traditional system for generating the seismic waves and recordingtheir reflections off the geological structures present in thesubsurface is illustrated in FIG. 1. A vessel 10 tows an array ofseismic receivers 11 provided on streamers 12. The streamers may bedisposed horizontally, i.e., lying at a constant depth relative to thesurface 14 of the ocean. The streamers may be disposed to have otherthan horizontal spatial arrangements. The vessel 10 also tows a seismicsource array 16 configured to generate a seismic wave 18. The seismicwave 18 propagates downward toward the seafloor 20 and penetrates theseafloor until eventually a reflecting structure 22 (reflector) reflectsthe seismic wave. The reflected seismic wave 24 propagates upward untilit is detected by the receiver 11 on the streamer 12. Based on the datacollected by the receiver 11, an image of the subsurface is generated byfurther analyses of the collected data.

However, this traditional configuration is expensive because of the highcosts associated with operating the towing vessel and the streamers. Inaddition, the data produced by the receivers of the streamers is poordue to the flow noise produced by the movement of the streamers inwater. Further, the notch diversity of the data recorded with thestreamers might be limited. To overcome some of these problems, newtechnologies deploy seismic sensors on the bottom of the ocean (oceanbottom stations, OBS) to achieve a coupling with the ocean bottom and toreduce the noise. Even so, positioning the seismic sensors remains achallenge for OBS technology.

Other technologies use permanent receivers set on the sea bottom, asdisclosed in U.S. Pat. No. 6,932,185 (herein '185), the entire contentof which is incorporated herein by reference. In this case, the seismicsensors 60 are attached, as shown in FIG. 2 (which corresponds to FIG. 4of the '185), to a heavy pedestal 62. A station 64 that includes thesensors 60 is launched from a vessel and arrives, due to its gravity, ata desired position. The station 64 remains on the bottom of the oceanpermanently. Seismic data recorded by sensors 60 is transferred througha cable 66 to a mobile station 68. When necessary, the mobile station 68may be brought to the surface to retrieve the seismic data.

Although this method provides a good coupling between the ocean bottomand the seismic receivers, the process is still expensive and notflexible because the stations and corresponding sensors are difficult tomove around or reuse. Further, positioning the stations is notstraightforward. Furthermore, the notch diversity is not greatlyimproved.

An improvement to this method is described, for example, in EuropeanPatent No. EP 1 217 390 (herein '390), the entire content of which isincorporated herein by reference. In this document, a receiver 70 isremovably attached to a pedestal 72 together with a memory device 74 asillustrated in FIG. 3. After recording the seismic signals, the receiver70 and the memory device 74 are instructed by a vessel 76 to detach fromthe pedestal 72 and to surface at the ocean surface 78 to be picked upby the vessel 76.

However, this configuration is not very reliable because the mechanismmaintaining the receiver 70 connected to the pedestal 72 may fail torelease the receiver 70. Also, the receiver 70 and pedestal 72 may notreach their intended positions on the seabed. Further, the fact that thepedestals 72 are left behind increases ocean pollution and the surveyprice, which is undesirable.

Thus, it can be seen from the above approaches that a characteristic ofthe existing methods is to record seismic signals either (i) close tothe surface, with streamers, or (ii) at the seabed with OBS. Neithersituation offers the desired notch diversity.

Accordingly, it would be desirable to provide systems and methods thatprovide an inexpensive and reliable device for recording seismic signalswith good notch diversity.

SUMMARY

According to an exemplary embodiment, there is a seismic survey systemfor recording seismic data underwater in the presence of underwatercurrents. The system includes first plural buoys configured to descendin water at a predetermined depth (H1) and each having a seismicreceiver for recording the seismic data; a first vessel configured tolaunch the first plural buoys along a first line; and a second vesselconfigured to recover the first plural buoys at a second line, whereinthere is a predetermined distance between the first and second lines.The first plural buoys are configured to travel underwater, atsubstantially the first predetermined depth (H1), from the first line tothe second line, due exclusively to the underwater currents.

According to another exemplary embodiment, there is a method forrecording seismic data underwater in the presence of underwatercurrents. The method includes determining trajectories of the underwatercurrents based on historic data; selecting a starting line substantiallyperpendicular to the underwater currents; launching, along the startingline, first plural buoys from a first vessel, the first plural buoysbeing configured to descend in water and each having a seismic receiverfor recording the seismic data; selecting a finish line substantiallyperpendicular on the underwater currents; and recovering, along thefinish line, the first plural buoys with a second vessel, wherein thereis a predetermined distance between the start and finish lines. Thefirst plural buoys are configured to travel underwater, at substantiallya first predetermined depth (H1), from the start line to the finishline, due exclusively to the underwater currents.

According to still another exemplary embodiment, there is a buoy forrecording seismic signals while underwater. The buoy includes a body; abuoyancy system configured to control a buoyancy of the body to descendto a predetermined depth (H1); a seismic sensor located on the body andconfigured to record the seismic signals; and a control deviceconfigured to maintain the body substantially at the predetermined depth(H1) while the buoy travels following an underwater current, and also toinstruct the seismic sensor to record seismic signals while travelingunderwater substantially parallel to a surface of the water.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate one or more embodiments and,together with the description, explain these embodiments. In thedrawings:

FIG. 1 is a schematic diagram of a conventional seismic survey system;

FIG. 2 is a schematic diagram of a station that may be positioned on thebottom of the ocean for seismic data recording;

FIG. 3 is a schematic diagram of another station that may be positionedon the bottom of the ocean for seismic data recording;

FIG. 4 is a schematic diagram of a seismic survey system that uses buoysfor recording seismic data according to an exemplary embodiment;

FIG. 5 is a schematic diagram of two vessels that determine anunderwater position of a buoy according to an exemplary embodiment;

FIG. 6 is a schematic diagram of a seismic survey system that usesplural buoys distributed at various depths for recording seismic dataaccording to an exemplary embodiment;

FIG. 7 is a schematic diagram of a seismic survey system that usesplural buoys distributed along a curve for recording seismic dataaccording to an exemplary embodiment;

FIG. 8 is a schematic diagram of a recovery vessel that instructsvarious buoys to resurface for recovery according to an exemplaryembodiment;

FIG. 9 is a schematic diagram of a buoy configured to record seismicsignals while traveling underwater according to an exemplary embodiment;

FIG. 10 is a schematic diagram of a seismic survey system configured totake into consideration lateral currents when recovering buoys accordingto an exemplary embodiment;

FIG. 11 is a schematic diagram of a seismic survey system that includesplural launching vessels and plural recovery vessels according to anexemplary embodiment;

FIGS. 12-15 are schematic diagrams of another seismic survey systemaccording to an exemplary embodiment;

FIG. 16 is a flowchart of a method for performing a seismic survey withsubstantially stationary buoys according to an exemplary embodiment; and

FIG. 17 is a flowchart of a method for performing a seismic survey withmoving buoys according to an exemplary embodiment.

DETAILED DESCRIPTION

The following description of the exemplary embodiments refers to theaccompanying drawings. The same reference numbers in different drawingsidentify the same or similar elements. The following detaileddescription does not limit the invention. Instead, the scope of theinvention is defined by the appended claims. The following embodimentsare discussed, for simplicity, with regard to the terminology andstructure of a buoy having seismic sensors and being deployed from adeployment vessel. However, the embodiments to be discussed next are notlimited to buoys being deployed from a vessel, but may be applied toother devices that may include seismic sensors.

Reference throughout the specification to “one embodiment” or “anembodiment” means that a particular feature, structure or characteristicdescribed in connection with an embodiment is included in at least oneembodiment of the subject matter disclosed. Thus, the appearance of thephrases “in one embodiment” or “in an embodiment” in various placesthroughout the specification is not necessarily referring to the sameembodiment. Further, the particular features, structures orcharacteristics may be combined in any suitable manner in one or moreembodiments.

New technologies in marine seismic surveys need an inexpensive systemfor deploying to and recovering from the sea seismic receivers.According to an exemplary embodiment, such a seismic system includesmultiple buoys, each having one or more seismic sensors. The buoys areinitially stored on a launching vessel. The launching vessel launchesthe buoys at predetermined distances along a path of the vessel. Theseismic receivers may include at least one of a hydrophone, geophone,accelerometer, electromagnetic sensor, etc. The buoys are configured tocontrol their own buoyancy so that each buoy is capable of reaching apredetermined depth and then resurfacing on its own, when instructed. Aseismic source (acoustic and/or electromagnetic) may be towed by thesame vessel or another vessel. The seismic source may include pluralindividual sources that are distributed along a horizontal line, avariable-depth line, a curved line, a parameterized variable-depth line,etc. After performing the recording, according to an exemplaryembodiment, a recovery vessel approaches the buoys, instructs them toresurface and collects them.

However, there are certain areas where the underwater currents arestrong and, thus, the buoys are not stationary during the seismicsurvey. For this situation, the new technology takes advantage of theexisting underwater currents and allows the buoys to travel underwaterto follow the current's path. For this reason, the current's path (ortrajectory) may be estimated before launching the buoys so it is knownwhere to expect the buoys to arrive after a given time for collectionpurposes. The launching and recovery vessels may track the position ofthe buoys and update the current paths based on real-time measurements.The seismic receivers of the buoys are configured to record seismicsignals as the buoy travels from the launching vessel to the recoveryvessel. The seismic signals are time-stamped and associated withcorresponding three-dimensional (3D) positions (coordinates of the buoy)where the signals were recorded.

A seismic system assumed to operate underwater with low or no watercurrents is now discussed in more detail according to an exemplaryembodiment illustrated in FIG. 4. FIG. 4 illustrates a seismic surveysystem 100 that may include a launching vessel 102, a recovery vessel104 and plural buoys 106. Both vessels 102 and 104 may act as (i)recovery or (ii) launching or (iii) recovery and launching vessels.These vessels may be small boats with a low cost of operation.

The launching vessel initially stores the plural buoys 106. When thesurvey is started, the launching vessel 102 launches the buoys 106 witha certain horizontal space interval d. The horizontal space interval dmay be, for example, between 10 and 200 m. However, the value of thehorizontal space interval may vary according to the goals of the seismicsurvey. The system 100 may also include one or more source vessels 120that are configured to tow a seismic source 122.

The seismic source 122 is configured to generate a seismic wave 124. Areflected seismic wave 126 is recorded by the seismic receiver of thebuoy 106. The seismic receiver may include any known receiver orcombination thereof.

The buoys 106 are configured to sink to a predetermined depth H and thento maintain that depth until instructed to the contrary. Although thebuoys can sink all the way to the ocean bottom, it is envisioned toperform seismic acquisition data with the buoys floating in the water,thus, away from the ocean bottom. The depth for the seismic survey maybe between 200 and 300 m. However, other depths may be used according tothe goals of the seismic survey.

The launching vessel launches the buoys while moving along apredetermined path. The buoys, assuming that there is low or nounderwater current, tend to maintain their absolute position by simplycontrolling their buoyancy. While recording the seismic waves, the buoysalso listen for an acoustic signal from the recovery vessel thatindicates that the buoy needs to resurface. Upon receiving that acousticsignal, the buoy resurfaces and it is then collected by the recoveryvessel.

A distance D between the launching and recovery vessels may be in theorder of kilometers, for example, 10 km. Under this scenario, asillustrated in FIG. 5, the launching vessel 102 may have two acousticpingers 202 and 204 (that form an acoustic system) and the recoveryvessel 104 may have two acoustic pingers 206 and 208. The acousticpingers may be provided on sides of the vessels and may be configured touse their own frequencies (f1 to f4) so that a buoy 210 receives fourdifferent frequencies from the acoustic pingers. The acoustic pingersmay be configured to transmit a signal, for example, every 5 seconds,with a range of 5 km. Another system for localizing the buoys isdescribed with reference to FIG. 12.

The buoy may have an oscillator (to be discussed later) that keeps acopy of the 5-second transmitting clock. Thus, the buoy is configured torecord the time of arrival of the acoustic signals from the pingers.Combining that information with the depth information given by itspressure gauge and the positions of the pingers (provided, for example,by a differential global positioning system (DGPS), and optionally bythe pitch and roll of the boats), it is possible to reconstitute theabsolute position of each buoy at any desired time so that thetime-stamped recorded signals may be mapped to the actual positions ofthe buoy when the seismic signals were recorded. FIG. 5 shows controlunits 220A and 220B mounted on each vessel. The control units mayinclude, in addition to the traditional processor and memory, the DGPS.Other GPS-type units may be used. The absolute position of each buoy maybe calculated by the control units. The control units map tracesrecorded by a buoy with corresponding underwater positions of the buoyas determined by the acoustic systems.

The control units may also be distributed on the vessel and the buoy.While FIG. 5 shows the acoustic pingers mounted on two vessels, it ispossible to have the acoustic pingers mounted only on a single vessel.

Thus, the plurality of buoys shown in FIGS. 4 and 5 may be imagined asthe equivalent of one or more streamers (in terms of seismic datacollection), but with the advantage that there is no drag on thevessels, a depth range of the buoys is much enlarged comparative with areal streamer, and a distance d between the receivers may be adjusted asdesired. Also, the buoys may be instructed, before being launched fromthe launching vessel 204, to have various depths as illustrated in FIGS.6 and 7.

In this respect, FIG. 6 is a side view showing a system 300 thatincludes a launching vessel 302, a recovery vessel 304 and plural buoys306. Some buoys form a first layer 308, which is located at a firstdepth H1, and other buoys form a second layer 310, which is located at asecond depth H2, different from H1. Although FIG. 6 shows the buoysprovided at only two depths, those skilled in the art would appreciatethat the buoys may be provided at more than two depths.

FIG. 7 is a side view showing a system 400 that includes a launchingvessel 402, a recovery vessel 404, and plural buoys 406. The buoys arearranged in this embodiment along a curve 408. The curve 408 may be astraight line, a parameterized depth-varying curve (e.g., parabola,hyperbola, exponential, circle, etc.), or a combination of them. In theembodiment illustrated in FIG. 7, the curve 408 has a curved portion 410and a straight line portion 412. It is noted that the buoys notdistributed on the curve 408 are travelling to either reach their finalunderwater position (those closer to the launching vessel 402) or theirsurface position (those closer to the recovery vessel 404) for beingrecovered by the recovery vessel 404.

The recovery phase of the buoys is now discussed with regard to FIG. 8.FIG. 8 shows a system 500 that includes a launching vessel (not shown),a recovery vessel 502 and plural buoys 504. A set of buoys 506 is shownhaving a predetermined depth, while another set of buoys 508 is in theprocess of surfacing, and still another set of buoys 510 is already atthe water surface 512 waiting to be recovered by the recovery vessel502.

The recovery vessel generates an acoustic signal 514 with an appropriateacoustic signal generator 516. The acoustic signal 514 may have afrequency f5 different from the frequencies used by the pingers of thelaunching and recovery vessels. The acoustic signal 514 may be ashort-range acoustic signal and constitutes a command for the buoy tosurface. When a buoy receives the acoustic signal 514, the buoyactivates its buoyancy system (to be discussed later) to resurface. Theset of buoys 508 is in the middle of the resurfacing process. Once atthe water surface, each buoy of the set of buoys 510 activates aradio-frequency (RF) beacon (transmitter) 524 for sending a signal 520to the RF goniometer 522 of the recovery vessel 502. Thus, each buoy mayhave an RF transmitter 524.

Based on this information, the recovery vessel 502 determines theposition of each buoy and recovers them. The recovery process mayinclude bringing the buoys on a deck of the vessel. The structure of abuoy is now discussed with regard to FIG. 9.

FIG. 9 illustrates an exemplary buoy 900. The buoy 900 may have a body901 that includes a buoyancy system 902 configured to control thebuoyancy of the buoy. For example, the buoyancy system 902 may changethe effective density of the buoy. The density of any object isdetermined by its mass divided by its volume. The buoy 900 may keep itsmass constant, but altering its volume changes its density. To achievethis, for example, a hydraulic piston may be used to push, e.g., mineraloil out of the buoy and expand a rubber bladder at the bottom end of thebuoy. As the bladder expands, the buoy becomes less dense than theseawater and rises to the surface. Upon being launched from thelaunching vessel, the buoy withdraws the piston and descends to thedesired depth to record seismic signals.

This is one example for controlling the buoyancy of the buoy. Thoseskilled in the art would appreciate that other systems may be employedfor controlling the buoyancy of the buoy. In one application, thebuoyancy system may include a motor and a propeller to further controlthe speed and direction of the buoy.

Further, the buoy 900 may include one or more sensors 904, e.g., apressure gauge, for determining the pressure and/or temperature of theambient of the buoy, etc. A processor 906 may be connected to thesensors 904 and the buoyancy system 902 for coordinating the up and downmovement of the buoy. The processor 906 may also be configured tocontrol the vertical speed of the buoy by controlling the buoyancy ofthe buoy. For example, the processor may be configured to achieve afirst speed for a shallow depth and a second speed for higher depths.Also, the processor 906 may calculate the depth of the buoy based on thepressure readings from the sensor 904.

The processor 906 may also be connected to a battery 908, ahigh-accuracy oscillator or clock module 910, e.g., atemperature-controlled crystal oscillator (TCXO), a data storage device912 for storing the recorded seismic data, an inertial device 914, a GPS916 and a corresponding antenna 916 a, and an RF beacon 918 and acorresponding antenna 918 a, etc. The battery 908 may be any knownbattery. The module 910 is configured to provide an accurate time to theprocessor 906 for correctly time-stamping the recorded seismic data. Inone application, the module 910 is configured to sample every 2 ms theacoustic signal and time-stamp it. The module 910 may also record acompass direction. Based on the temperature sensor, the module 910 mayadjust/correct its oscillating time to provide an accurate time as thewater temperature is changing.

The optional inertial device 914 may be an inexpensive inertialnavigation system (INS). An inertial navigation system includes at leasta module containing accelerometers, gyroscopes or other motion-sensingdevices. The INS is initially provided with the position and velocity ofthe buoy from another source, for example, a human operator, the GPS916, etc., and thereafter the INS may compute its own updated positionand velocity by integrating information received from its motionsensors. The advantage of an INS is that it requires no externalreferences in order to determine its position, orientation or velocityonce it has been initialized. Further, usage of the INS is inexpensive.However, in the exemplary embodiment discussed herein, the position ofthe buoy is determined using the pingers of the vessels discussed above.

The buoy 900 may also include the RF beacon 918, which is configured tosend RF signals such that a vessel can locate the buoy. The processor906 is configured to activate the RF beacon 918 when the buoy is at thesurface of the water, or the antenna 918 a is capable of transmittingthe RF signals to a vessel. Those skilled in the art would recognizethat the buoy may include other equipment that helps the navigation.However, it is desirable to provide an inexpensive buoy and, for thisreason, the equipment added to the buoy should be kept to a minimum.

In terms of seismic equipment, the buoy 900 may include one or moreseismic sensors 920. Such a sensor may be at least one of a hydrophone,geophone, accelerometer, electromagnetic sensor, etc. In oneapplication, the seismic sensor includes only a hydrophone. In anotherapplication, the seismic sensor includes a hydrophone and threegeophones. Once the buoy has reached the desired depth, the buoystabilizes its position by performing, for example, a control loopbetween the buoyancy control system and the pressure gauge.Additionally, the buoy 900 may include an acoustic signal conditioningmodule 922. This module is configured to process the acquired seismicsignals, for example, to apply various filters to the recorded seismicsignals.

The embodiments discussed above have assumed that the water currents areminimal or non-existing and, thus, the buoy maintains its position in aplane substantially parallel to the water surface without additionaldevices. However, if there are some underwater currents (not strongcurrents) that need to be taken into account, the next exemplaryembodiment explains how to address this matter. Still with regard toFIG. 9, the buoy 900 may include a propulsion system 930. At a minimum,the propulsion system 930 may include a motor 932 and a propeller 934.The propeller 934 is show in the figure configured to move the node on avertical direction Z. However, the propeller 934 or additionalpropellers may be oriented to provide movement in the XY plane for thebuoy 900. Thus, the buoy would have a dynamic control to stay at theirposition, e.g., a propulsion system capable to adjust the position ofthe node in the XY plane and a buoyancy control for its depth. Theprocessor 906 may be connected to the propulsion system 930 fordynamically maintaining the position of the buoy, when deployed, at thedesired target position.

To maintain the target position, the buoy may have a system (e.g.acoustic system, USBL (to be discussed later), pressure gauge, etc.)which permits them to know where they are and control their position.

FIG. 9 also shows a communication interface 940 that is capable ofexchanging data with a system on the vessel, for transferring therecorded seismic data when the buoy is retrieved on the vessel. Thecommunication interface 940 may be wired or wireless, e.g., a wi-fiinterface. Other known types of wireless interfaces may be used.

FIG. 10 shows a top view of a system 1000 in which the launching vessel1002 launches buoys 1004 a-e along a predetermined path 1006. Areal-time map of the buoys may be achieved by using the pingers of thevessels. Thus, a controller 1007 provided on the recovery vessel 1008,or on the launching vessel 1002, or distributed on both vessels, maycalculate how strong the underwater currents 1010 are and may instructthe recovery vessel 1008 to take a path 1012 to correctly intercept theresurfacing buoys.

The buoys may also be used to perform a 3D seismic survey as shown inFIG. 11. The system 1100 may include plural launching vessels 1102 a-eand corresponding plural recovery vessels 1106 a-e. Buoys 1104 arelaunched by each launching vessel and recovered by the correspondingrecovery vessel as explained in the previous embodiments. Source vesselsmay be used to obtain a wide azimuth seismic survey. Thus, such a systemmay work similarly to a conventional system in which a vessel towsmultiple streamers. In this case, a “streamer” is formed by the buoysbetween the launching vessel and the recovery vessel.

However, the system shown in FIG. 11 is cheaper than the conventionalstreamer system because the vessels used to launch and recover the buoysare not as sophisticated as the vessel that tows the streamers, there isno drag produced by the buoys on the vessel, and the buoys themselvesare cheaper than the streamers. In addition, the present system obtainsmore diversified data, the seismic receivers on the buoys reach agreater depth than the current depths achieved by the streamers, and theflow noise is minimized or entirely suppressed because the speed of thebuoy during data acquisition is substantially zero.

In addition, because the flow noise present in the case of the realstreamers is absent in the present design, the speed of the launchingand recovery vessels may be increased above the conventional 5 knots perhour used in streamer-based seismic surveys. This decreases the timenecessary for completing the survey, which results in a reduced cost forrenting and operating the equipment, and also reduces personnelexpenses. Further, the present system may be deployed near obstructedareas, e.g., next to drilling platforms, etc. Not the least, the datarecorded with the present system achieves the highest notch diversity,which is desirable for data deghosting.

However, if the underwater currents are significant, the above-discussedembodiments pose a challenge to the recovery vessel when recovering thebuoys because the buoys may spread beyond a desired range. Thus,according to another exemplary embodiment, the high underwater currentsmay be used to the advantage of the seismic survey as now discussed.

As illustrated in FIG. 12, a system 1200 includes a launching vessel1202, a recovery vessel 1204, and plural buoys 1206. The vessels may beequipped with an acoustic system 1210 for tracking the position of thebuoys when underwater. An exemplary acoustic system is described next,and this system can track the buoy with an accuracy of approximately 5 mat a distance in the order of kilometers.

The acoustic system 1210 may be an Ultra-short baseline (USBL) system,also sometimes known as Super Short Base Line (SSBL). This system uses amethod of underwater acoustic positioning. A complete USBL systemincludes a transceiver, which is mounted on a pole under a vessel, and atransponder/responder on the buoy. A processor is used to calculate aposition from the ranges and bearings measured by the transceiver. Forexample, an acoustic pulse is transmitted by the transceiver anddetected by the subsea transponder, which replies with its own acousticpulse. This return pulse is detected by the transceiver on the vessel.The time from the transmission of the initial acoustic pulse until thereply is detected is measured by the USBL system and is converted into arange. To calculate a subsea position, the USBL calculates both a rangeand an angle from the transceiver to the subsea buoy. Angles aremeasured by the transceiver, which contains an array of transducers. Thetransceiver head normally contains three or more transducers separatedby a baseline of, e.g., 10 cm or less.

FIG. 12 also shows a seismic survey area 1220 that is desired to besurveyed with the buoys 1206. Based on various data (e.g., historicdata) available for the underwater currents corresponding to the area1220, a current model is developed through computer calculations in acomputer system 1222. The output of these computer calculations is anumber of current paths/trajectories 1224. Thus, a current map 1226 maybe developed for the area 1220 of interest. Alternatively or inaddition, the current map 1226 may be generated by using the positionsof the buoys 1206. For example, as the buoys 1206 travel for some time,the acoustic system 1210 may monitor their positions and based on thisinformation, infer the current map. With this capability, the currentmap may be updated while the seismic survey is performed and, thus, thetrajectories of the buoys may be updated. Consequently, the line 1220Bwhere the vessel 1204 waits to retrieve the buoys may change during thesurvey. With this information, the survey is designed so that the buoysare launched at a side (line) 1220A of the area 1220 of interest suchthat the underwater current 1224 would take the buoys to the oppositeside (line) 1220B of the area 1220. Thus, a course/trajectory of thelaunching and/or recovery vessels may be substantially perpendicular onthe trajectories of the underwater currents.

A distance 1220C travelled by the buoys may be in the order ofkilometers, e.g., 20 to 30 km. In other words, the buoys are expected inthis embodiment to travel along the current 1224, from one side 1220A ofthe survey area 1220 to an opposite side 1220B of the survey area 1220.

Thus, for such an arrangement, the launching vessel is instructed totravel back and forth along the first side 1220A and to launch a firstset of buoys 1206A at a first pass, a second set of buoys 12068 at asecond pass (later in time) as shown in FIG. 13, and so on until adesired number of buoys has been launched as shown in FIG. 14. It isnoted in these figures the path 1230 followed by the launching vesseland how the buoys are spaced along an X direction at a desired firstdistance and along a Y direction at a desired second distance. FIG. 14also shows how the first set of buoys 1220A has reached the recoveryvessel 1204. It is noted that the buoys 1220 are distributed underwatersimilar to the embodiment shown in FIG. 4, with the exception that inthat embodiment the buoys are almost stationary, while in FIGS. 12-14the buoys travel underwater, up to tens of kilometers as required by thesurvey. For example, for a current of 700 m/h as might be in the Gulf ofMexico, a buoy may travel along the current 1224 for 84 km in five days.Further, the buoys may be configured to attain different depths, e.g., afirst wave of buoys may float at a first depth H1 and a second wave ofbuoys may float at a second depth H2. In another application, the depthsof the successive waves may increase up to a point so that the array ofbuoys resemble plural streamers having a depth-varying profile. Thedepth-varying profile may be a slanted line or a parameterized curve. Inone application, the depth-varying profile extends parallel with lines1220A and/or 1220B while in another application the depth-varyingprofile extends parallel with distance 1220C.

If there is a desire to limit the spread of the buoys over the X axis,the survey may be divided into smaller areas (rectangles or othershapes) 1220-1 to 1220-3 as shown in FIG. 15, and each smaller area maybe surveyed as shown in FIGS. 12-14. Alternatively, the buoys may beprovided with the propulsion system discussed above and it may be usedto prevent the spread of the buoys. Once the survey of one area isfinalized, the recovery vessel may become the launching vessel and theformer launching vessel moves to the end of the second area to restartthe recovery for the survey.

The above-discussed embodiments may be implemented as methods as nowdiscussed. According to an exemplary embodiment illustrated in FIG. 16,there is a method for recording seismic data underwater with a seismicsurvey system. The method includes a step 1600 of launching first pluralbuoys from a first vessel, the first plural buoys being configured todescend at a first predetermined depth (H1) in water, at least one buoy(210) having a seismic receiver for recording the seismic data; a step1602 of generating seismic waves with a source; a step 1604 of recordingreflected seismic waves with the seismic receiver while the at least onebuoy maintains its first predetermined depth (H1); a step 1606 ofdetermining a position of the at least one buoy with a first acousticsystem attached to the first vessel and with a second acoustic systemattached to the second vessel; and a step 1608 of recovering the atleast one buoy with the second vessel. The at least one buoy isinstructed to stay at the first predetermined depth (H1) underwaterwhile recording the seismic data.

According to another exemplary embodiment illustrated in FIG. 17, thereis a method for recording seismic data underwater in the presence ofunderwater currents. The method includes a step 1700 of determiningtrajectories of the underwater currents based on historic data; a step1702 of selecting a starting line substantially perpendicular on theunderwater currents; a step 1704 of launching, along the starting line,first plural buoys from a first vessel, the first plural buoys beingconfigured to descend in water and each having a seismic receiver forrecording the seismic data; a step 1706 of selecting a finish linesubstantially perpendicular on the underwater currents; and a step 1708of recovering, along the finish line, the first plural buoys with asecond vessel configured, wherein there is a predetermined distancebetween the start and finish lines. The first plural buoys areconfigured to travel underwater, at substantially a first predetermineddepth (H1), from the start line to the finish line, due to theunderwater currents.

The systems and processes discussed above are just some examples forillustrating the novel concepts of using buoys for seismic datarecording. Those skilled in the art would appreciate that these systemsand/or processes may be changed, adjusted or modified to fit variousneeds. For example, the buoys may be replaced with similar devices thatuse no propelling means for reaching a desired depth.

In this regard, it is noted that it is known in the field to useautonomous underwater vehicles (AUV) for deploying seismic sensors.However, an AUV is different from a buoy in the sense that the buoy doesnot have a propulsion system, i.e., motor and associated propeller orwater pump. Another difference between existing AUVs and the presentbuoys is that AUVs travel to the seabed and back without recordingseismic data. Conventional AUVs land on the seabed and make some seismicrecordings while stationary, after which they return to the surface.

One or more of the exemplary embodiments discussed above disclose a buoyconfigured to perform seismic recordings. It should be understood thatthis description is not intended to limit the invention. On thecontrary, the exemplary embodiments are intended to cover alternatives,modifications and equivalents, which are included in the spirit andscope of the invention as defined by the appended claims. Further, inthe detailed description of the exemplary embodiments, numerous specificdetails are set forth in order to provide a comprehensive understandingof the claimed invention. However, one skilled in the art wouldunderstand that various embodiments may be practiced without suchspecific details.

Although the features and elements of the present exemplary embodimentsare described in the embodiments in particular combinations, eachfeature or element can be used alone without the other features andelements of the embodiments or in various combinations with or withoutother features and elements disclosed herein.

This written description uses examples of the subject matter disclosedto enable any person skilled in the art to practice the same, includingmaking and using any devices or systems and performing any incorporatedmethods. The patentable scope of the subject matter is defined by theclaims, and may include other examples that occur to those skilled inthe art. Such other examples are intended to be within the scope of theclaims.

What is claimed is:
 1. A seismic survey system for recording seismic data underwater in the presence of underwater currents, the system comprising: first plural buoys configured to descend in water at a predetermined depth (H1) and each having a seismic receiver for recording the seismic data; a first vessel configured to launch the first plural buoys along a first line; and a second vessel configured to recover the first plural buoys at a second line, wherein there is a predetermined distance between the first and second lines; wherein the first plural buoys are configured to travel underwater, at substantially the first predetermined depth (H1), from the first line to the second line, due exclusively to the underwater currents.
 2. The system of claim 1, wherein the first vessel is configured to launch second plural buoys along the first line, later in time than the first plural buoys.
 3. The system of claim 2, wherein the first vessel is configured to launch waves of buoys to cover a desired area for collecting the seismic data.
 4. The system of claim 2, wherein the second vessel is configured to move back and forth along the second line to recover the second plural buoys.
 5. The system of claim 1, wherein the predetermined distance is larger than 10 km.
 6. The system of claim 1, further comprising: a computer system configured to calculate the underwater currents prior to launching the first plural buoys and to determine a position of the second line, wherein the computer system calculates the underwater currents based on historic data.
 7. The system of claim 6, wherein the computer system is configured to receive current positions of the first plural buoys, and to calculate new trajectories of the underwater currents based on the current positions of the first plural buoys.
 8. The system of claim 1, further comprising: second plural buoys configured to descend at a second predetermined depth in water.
 9. The system of claim 1, further comprising: second plural buoys configured to descend in water to form a variable-depth profile.
 10. A method for recording seismic data underwater in the presence of underwater currents, the method comprising: determining trajectories of the underwater currents based on historic data; selecting a starting line substantially perpendicular to the underwater currents; launching, along the starting line, first plural buoys from a first vessel, the first plural buoys being configured to descend in water and each having a seismic receiver for recording the seismic data; selecting a finish line substantially perpendicular on the underwater currents; and recovering, along the finish line, the first plural buoys with a second vessel, wherein there is a predetermined distance between the start and finish lines, wherein the first plural buoys are configured to travel underwater, at substantially a first predetermined depth (H1), from the start line to the finish line, due exclusively to the underwater currents.
 11. The method of claim 10, further comprising: launching from the first vessel second plural buoys along the start line, later in time than the first plural buoys.
 12. The method of claim 11, further comprising: launching waves of buoys from the first vessel to cover a desired area for collecting the seismic data.
 13. The method of claim 12, further comprising: moving the second vessel back and forth along the finish line to recover the second plural buoys.
 14. The method of claim 10, wherein the predetermined distance is larger than 10 km.
 15. The method of claim 10, further comprising: calculating with a computer system the underwater currents prior to launching the first plural buoys; and calculating a position of the finish line.
 16. The method of claim 15, further comprising: receiving current positions of the first plural buoys; and calculating new trajectories for the underwater currents based on the current positions of the first plural buoys.
 17. The method of claim 16, further comprising: determining the current positions of the first plural buoys based on an acoustic system installed at the first and/or second vessels.
 18. The method of claim 10, further comprising: launching second plural buoys that descend at a second predetermined depth in water.
 19. The method of claim 10, further comprising: launching second plural buoys to descend in water to form a variable-depth profile.
 20. A buoy for recording seismic signals while underwater, the buoy comprising: a body; a buoyancy system configured to control a buoyancy of the body to descend to a predetermined depth (H1); a seismic sensor located on the body and configured to record the seismic signals; and a control device configured to maintain the body substantially at the predetermined depth (H1) while the buoy travels following an underwater current, and also to instruct the seismic sensor to record seismic signals while traveling underwater substantially parallel to a surface of the water. 